Generally, the hydraulic drive vehicle such as a wheeled hydraulic excavator comprises a hydraulic pump and a hydraulic motor for travelling which is driven by oil discharged from the hydraulic pump. The output shaft of this hydraulic motor is connected with the input shaft of the transmission, and the rotation of the hydraulic motor is transmitted to the wheels through the transmission. A drain chamber is provided to the hydraulic motor, and the drain oil from the hydraulic motor returns to a reservoir via the drain chamber.
In such a hydraulic drive vehicle as described above, if a foreign body should be ingested by the hydraulic motor, proper operation of the hydraulic motor is impeded and there is a danger that the hydraulic motor may be damaged. If the hydraulic motor is damaged, the discharged oil from the hydraulic pump flows into the drain chamber and in some cases it may flow into the transmission. As a result, the transmission therein is filled with the oil, and a great resistance comes to act on the transmission so that the travelling performance of the vehicle deteriorates. Moreover, when transmission oil becomes mixed with the oil from the hydraulic motor, the quality of the mission oil may be deteriorated, and this may exert a negative influence upon the operation of the transmission.